The present invention relates to the production of components by welding. The invention relates more particularly to a process for welding metal fiber by capacitor discharge in order to produce components of required shape.
Existing capacitor discharge welding processes are known (GB 1 508 350). These processes consist in passing a current, generally obtained by discharging a capacitor, through particles of metallic material so as to weld them together. These processes have been applied to powders of spherical particles (P. A. Vityaz et al, xe2x80x9cContact formation during the electric pulse sintering of a titanium alloy powderxe2x80x9d, Belorussian Republican Powder Metallurgy Scientific Production Association, translated by Poroshkovaya Metallurgiya, No. 7 (331), pages 20-23, July 1990) or of elongate particles such as fibers (S. T. S. Al Hassani et al, xe2x80x9cPreforming using high-voltage electrical dischargexe2x80x9d, Powder Metallurgy, No. 1, page 45, 1980).
These processes have sometimes also been combined with the application of pressure (R. W. Boesel et al, xe2x80x9cSpark sintering tames exotic P/M materialsxe2x80x9d, Materials Engineering, page 32, October 1969), so as to facilitate the welding and eliminate as far as possible the porosity of the components thus welded. These components are compact and their level of porosity is close to 0 (if Vm is the volume of material and Vc is the volume of the finished component, the degree of porosity xcfx84 is defined as xcfx84=1 xe2x88x92(Vm/Vc)).
In contrast, the disclosed invention is aimed at the manufacture of porous components. These components may, for example, be supports for an active material, such as the fibrous structures for catalytic converters.
It is necessary for these components to have a very high level of porosity, allied with excellent mechanical strength over a wide temperature range.
The desired levels of porosity start from 0.60 and typically are in the region of 0.95. The level varies according to the shape and the function of the components to be produced.
Finally, the manufacture of these components must also be controlled so as to achieve good reproducibility with precise dimensions.
This type of application therefore produces specific problems to which the process of the invention and the apparatus for implementing it prove a solution.
The invention is a process for forming metal components of controlled porosity by welding, comprising the known successive steps consisting of:
preparing a predetermined amount of metal elements of anistropic geometrical shape, intended for constituting a component;
distributing this predetermined amount of metal elements in a mold having at least one movable part;
exerting pressure using a movable part of the mold controlled by an external means, which movable part possibly constituting an electrode, in at least one main direction on this predetermined amount of metal elements, said pressure being intended to reinforce and maintain the points of contact between these elements;
simultaneously passing an electric current through this predetermined amount of metal elements via a set of two electrodes of opposite polarity in order to join these metal elements together by welding, said two electrodes being placed so that the direction of flow of the current is overall coaxial with said main direction of the pressure exerted on the predetermined amount of metal elements; and
removing the component from the mold.
The expression xe2x80x9celements of anisotropic geometrical shapexe2x80x9d is understood to mean articles having at least one of the three dimensions significantly different from the other or others.
The process of the invention is characterized in that:
the predetermined amount of metal elements is obtained by weighing a mass of metal elements whose value M is defined as a function of the desired degree of porosity xcfx84, the volume of the component Vc and the density of the metal alloy used xcfx81a by the formula:
M=VC xcfx81a(1 xe2x88x92xcfx84)
the predetermined amount of metal elements is distributed isotropically in the mold;
the pressure exerted is progressively increased until the component has the required shape, thus giving the component the desired level of porosity; and
the movable part of the mold is then held in position and, simultaneously, the electric current flows through the metal elements and welds them together by local melting at the points of contact due to the Joule effect or by creation of a local arc.
The expression xe2x80x9clocal melting at the points of contactxe2x80x9d is understood to mean melting relating to only part of each of the cross sections in three dimensions of the metal elements. This melting is such that, on the one hand, the mechanical strength of each metal element in question, although momentarily reduced, remains sufficient for all of these elements to retain the shape acquired during the previous step, thus retaining the isotropic distribution in the mold, and, on the other hand, the mechanical strength of the component is optimal for the use.
The elements of anisotropic geometrical shape of the invention preferably have one dimension significantly different from the other two. They are therefore generally oblong and advantageously are in the form of needles, flakes or nonwoven fibers.
It is very desirable for easy implementation of the process for the elements to distribute themselves spontaneously in an isotropic manner in the mold. Elements of approximately cubic or spherical shape for example distribute themselves spontaneously in an isotropic manner in a mold. However, these elements are not anisotropic. Their use in the process of the invention does not allow the desired level of porosity to be achieved (xcfx84max =0.5 in the case of tubes and 0.48 in the case of spheres).
However, elements having both an anisotropic geometrical shape and the ability to distribute themselves spontaneously in an isotropic manner in a mold do exist. Such elements are obtained in particular by the technique of casting on a wheel. In fact, the elements produced using this technique have, among other characteristics, the particular feature of having surface asperities, mainly on the edges parallel to the significantly different dimension. These asperities prevent the elements from sliding against one another and thus prevent them from being distributed anisotropically under the effect of gravity.
Thus, they distribute themselves isotropically in the mold without any further manipulation such as, for example, shaking them. The level of porosity spontaneously obtained may be up to 0.99, which value may be greater than that of the desired level of porosity. To maintain the isotropic character of the fiber distribution in the component, the spontaneous level of porosity must remain close to that desired. For this purpose, the metal elements may be ground or chopped beforehand so as to size them according to the significantly different dimension with a suitable value.
It is therefore by applying pressure to the metal elements by means of a movable part of the mold that the required shape is given to the component and, likewise, the desired level of porosity. The applied pressure progressively increases up to the necessary value so that the movable part of the mold reaches the position corresponding to the required shape. There is therefore a balance between the force exerted by the external means and the elastic reaction force from the compressed elements.
The movable part is then held in position. It should be understood by this that the movable part of the mold can no longer change position, even if the reaction force exerted by the compressed elements suddenly varies.
This is because, when the electric current flows through the metal elements, the local melting causes the force exerted by these elements on the movable part of the mold to suddenly be decreased. If the force from the external means is kept constant and if the movable part is left free in position, this results in strong compression and deformation of the component owing to the imbalance in the forces.
With the movable part held in position, it is necessary to deliver an electric current flowing through the metal elements such that it allows local melting, as defined above, but without causing complete melting at the contact points, this melting being defined as melting over the entire cross section of the metal elements. This is because, if the current delivered is too high, complete melting of many elements takes place and, by gravity, this results in deformation of the component.
The electric current thus controlled is advantageously delivered by an electrical generator using a capacitor of capacitance C, which constitutes an economic, simple and well-suited means for this type of application.
The apparatus according to the invention comprises a set of electrodes, at least one of which is fastened to a movable wall.